1. Field of the Invention
The invention pertains to the field of reception from direct broadcasting TV satellites and, more particularly, to a reception antenna system.
2. Description of the Prior Art
In the field of reception from direct broadcasting satellites (D.B.S.) such as TDF 1, TV SAT, OLYMPUS, BSD etc., different sorts of known antennas may be used.
A first type of known antenna uses a paraboloid of revolution, with a source placed in the focal point of this paraboloid. The antenna access is obtained either directly at the focal point of the parabola, at the source access, or in the rear of the antenna, a crosshead formed by a guide making the connection between the source and the rear of the antenna. The antenna is placed on a support dimensioned as a function of the size of the parabola. This support enables aiming, in azimuth and elevation, towards the satellite to be received.
The drawback of this type of antenna is that the shadow projected from the source, from its support and from its holding arms masks a part of the reflector: this causes a reduction in efficiency. Moreover, the use of a waveguide to have access to the rear of the antenna makes it possible to protect the low-noise converter (LNC) but causes a loss of transmission that implies a reduction in gain and an increase in the noise temperature of the antenna. Furthermore, this is a high-cost type of antenna, notably because of the number of mechanical parts that have to be used to make the antenna structure and enable its orientation. The latest developments in this type of antenna have made it possible, by means of GaAs FETs, to obtain small-sized low-noise converters which it has been possible to place directly behind the source, at the focal point of the paraboloid, so as to reduce the transmission losses. But, in doing this, the mask on the reflector is further increased, notably for small-diameter antennas. Furthermore, the electronic circuitry is then more directly subjected to climatic conditions, especially variations in temperature, and to the vibrations induced.
Another type of known antenna uses an off-centered parabolic reflector and is commonly called an off-set antenna. The reflector of this antenna is formed by a portion of a paraboloid of revolution. The source, which is away from the axis of this paraboloid, projects no shadow on the aperture. To this end, the reflector is obtained by cutting out a paraboloid in a cylinder with a diameter D, centered on an axis parallel to the focal axis of the paraboloid. The source is then placed at the focus F of the paraboloid and is aimed at the middle of the paraboloid portion. The antenna access is generally obtained at the source access, and the low-noise converter, in this case, is placed directly behind the source, before the reflector.
The chief advantage of this "off-set" structure is the increase in the efficiency of the antenna through a reduction in the mask effect of the source. Moreover, the antenna has little sensitivity to climatic conditions and, by its structure, the antenna aimed towards the satellite is practically vertical. However, this second type of antenna also has major drawbacks: the making of this type of reflector, which is not of a shape generated by revolution, is difficult and little suited to fabrication by metal-spinning or drawing. Moreover, the radiation patterns of the antenna are not generated by revolution and the rate of ellipticity of an antenna such as this, used in circular polarization, is higher than with an antenna using a reflector in the shape of a paraboloid of revolution. Besides, the low-noise converter is placed before the reflector, and is therefore subjected to climatic conditions (temperature in particular). Finally, since the reference plane in elevation is not easily defined, it is not easy to aim the antenna in the direction of the satellite to be received.